Several devices are commercially available which already feature similar functions, although obtained by other means. A typical approach would consist in placing a magnetic field sensor in an airgap whose geometry changes as a function of the angular or linear displacement to be measured. Said change of geometry is arranged as to result in a corresponding change in the value of the magnetic field, B, as measured by said magnetic field sensor.
The change in the B field value can be obtained by either varying the length of the airgap, or its cross-section, or both, as a function of the displacement in position. The straight implementation of such basic approaches is rather obvious, being just a matter to devise magnetic circuit configurations such that the value of B is a direct function of the relative position of the composing parts. The simplest and most robust way to generate the required magnetic field would then make use of permanent magnets. Unfortunately things are not that straightforward, as the working point of low cost permanent magnets (PMs) is heavily affected by temperature, by naturally occurring demagnetization effects, and variations in magnetic circuit's total reluctance. To counteract the inaccuracies inherent to said trivial implementations, more sophisticated techniques have been developed, as disclosed in numerous patents.
In particular, EP 0 768 541 A1 (referred to as D1), “Capteur Magnétique de Position”, SAGEM SA, 01.10.1997, discloses a magnetic circuit configuration arranged as to generate two magnetic fields, B1 in airgap 1, and B2 in airgap 2, each one of which is a linear function of position, and whose sum is a constant value. The position is then measured by computing the value of the relative differential measurement (B1−B2)/(B1+B2), greatly reducing sensitivity to variations in temperature.
For magnetic type of contactless position measurements an additional source of inaccuracy are external stray fields, it is hence important to try to minimise their relative importance. U.S. Pat. No. 5,789,917 A1 (referred to as D2), “Magnetic Position Sensor with Hall Probe Formed in an Air Gap of a Stator”, Moving Magnet Technologies SA, Aug. 4, 1998, discloses a magnetic circuit configuration arranged as to effectively screen from external fields.
For the measurement of the value of the magnetic field, both D1 and D2 make use of Hall effect probes. Hall effect probes generate an output voltage, Vh (Volts), proportional to the value of the biasing current, Ib (Amperes), and to the magnetic field, B (Tesla), through the factor of proportionality, Kh (sensitivity, with units V/AT):Vh=Kh*Ib*B  (1)